Controlled-source electromagnetic (“CSEM”) geophysical surveys use active (man-made) sources to generate electromagnetic fields to excite the earth, and deploy receiver instruments on the earth's surface, the seafloor, or inside boreholes to measure the resulting electric and magnetic fields, i.e., the earth's response to the source excitation. FIG. 1 illustrates the basic elements of an offshore CSEM survey. A vessel tows a submerged CSEM transmitter 11 over an area of sub-sea floor 13. The electric and magnetic fields measured by receivers 12 are then analyzed to determine the electrical resistivity of the earth structures (subsurface formations) beneath the surface or seafloor.
CSEM surveys are an important geophysical tool for evaluating the presence of hydrocarbon-bearing strata within the earth. These surveys typically record the electromagnetic signal induced in the earth by a source (transmitter) and measured at one or more receivers. The behavior of this signal as a function of transmitter location, frequency, and separation (offset) between transmitter and receiver can be diagnostic of rock properties associated with the presence or absence of hydrocarbons. A notable diagnostic rock property of this kind is electrical resistivity. Thus, CSEM measurements are typically used to determine the spatially-varying resistivity of the subsurface.
As can be appreciated, this technology has been applied for onshore mineral exploration, oceanic tectonic studies, and offshore petroleum and mineral resource exploration. For example, as noted above, the controlled-source electromagnetic (“CSEM”) geophysical survey may be performed on vehicles in a land-based system, in the vessels in water-based systems and/or by aircraft by air-based devices, which are further discussed in various documents. See, for example, Chave et al., Electromagnetic Methods in Applied Geophysics (ed. M. N. Nabighian) 2, 931-966, Society of Exploration Geophysicists (1991); Constable and Cox, J. Geophs. Res., Vol. 101, 5519-5530 (1996); MacGregor et al., Geophy. J. Int. 146, 217-236 (2001); S. Ellingsrud et al., The Leading Edge, 972-982 (2002); Eidesmo et al., First Break 20.3, 144-152 (2002).
The electromagnetic (“EM”) fields generated by the transmitter may be created by injecting the currents into the earth or seawater/seafloor or by oscillating the currents in closed-loop wire, in either case, using a chosen periodic or transient waveform. The shape of the transmitted waveform determines its frequency spectrum. That is, the transmitter, through its waveform, controls the frequency content, frequency distribution and amplitude at each frequency for the transmitted waveform. These measured electric and magnetic fields are then analyzed to determine the electrical resistivity of the earth structures beneath the earth's surface or seafloor.
One type of transmitter waveform is the time division source waveform, which is a compound waveform formed by alternating two or more base waveforms, each chosen primarily for its different fundamental frequency so as to collectively provide the desired range of frequencies. Typically, each base waveform is repeated N times, which is called herein a sub-sequence of the compound waveform. FIG. 2 shows an example having six square-wave sub-sequences. This type of compound waveform is discussed in Patent Application Publication US 2008/008920, by Lu, Hornbostel, and Willen). The main advantages of time division waveforms are twofold: the frequency content can be specified with more flexibility; and the frequency components are localized in time leading to a possibly improved signal-to-noise ratio (SNR).